12_optimizer: create docments

[warner-benjamin]

Add docments to 12_optimizer.

See the style guide and contribution guide for more details (links will be added when created).

This requires advanced to expert familiarity with fastai, pytorch, and optimizers.

Docments progress:

• [x] _BaseOptimizer
• [x] Optimizer
• [ ] Basic steppers
• [ ] Statistics
• [ ] SGD with momentum
• [ ] RMSProp
• [ ] Adam
• [ ] RAdam
• [ ] QHAdam
• [ ] LARS/LARC
• [ ] LAMB
• [ ] Lookahead
• [ ] OptimWrapper

[tyoc213]

@warner-benjamin sorry, don't know why I commented in the other ticket, but I was referring to this one, only for _BaseOptimizer and Optimizer

[tyoc213]

I guess some one can continue with the other parts? wdyt @tmabraham @warner-benjamin ?